* The Digestive System
* Urinary Tracts

Picture source: Gezondheidsnieuws

The digestive System

The entire digestive tract (esophagus, stomach, liver, pancreas, intestine and rectum) has a length of about 9 meters. Every day about 11 liters of food, liquids and digestive juices run from the digestive tract. Our digestive system is unique: it can digest meat, vegetables and cereals. It normally takes a meal 24 - 30 hours to go through completely. It spends the most time in the large intestine and the rectum. Ultimately about 1.5 liters of water, along with the indigestible food, goes into the large intestine. The stools contain just a deciliter (100mls) of water. An average adult has 100 - 150 grams a day of stools. During a lifetime the average gut has 65,000 pounds of food and drink to process (the weight of 12 elephants!).

It begins in the mouth (cavum oris) where the food is chewed and mixed with saliva. In the saliva are enzymes, which help digest food. Salivary glands produce saliva not only as if food enters the mouth, but also if you see or smell food, and even if you think about it. By day you produce about 1200 ml of saliva. The composition depends on the type of food. For a good digestion you should chew each bite 40 times. After swallowing, the nasal cavity is closed by the uvula and the trachea by the throat cover, the food goes via the throat (pharynx) into the gullet (esophagus), which is a 25 cm long flexible tube with a diameter of 2.5 cm. From here the body regulates the digestion itself: you do not consciously do it. The food very quickly goes through the esophagus, on its way to the stomach (ventricles, gaster). Because of the peristaltic movements of the esophagus, you can still eat and drink if you lie down, or even if you are standing on your head.

Nutrients are absorbed in the small intestine. As the intestine makes kneading movements the nutrients can already be absorbed by the blood through the intestine wall. The peristaltic movements push the chime through the small intestine at a speed of more than one centimetre per second in approximately 4.5 hours. At the end of the small intestine, a length of some 6 to 7 metres, almost all the useful nutrients have been absorbed.

The inside of the small intestine is covered with what are known as villi, sort of tiny projections which
ensure that the surface where the nutrients can be absorbed is considerably increased.

Picture source: from the book "Atlas van de anatomie".

The rest of the food than goes to the 1.5 meter long large intestine (cartel intestine, or colon interstinum crassum), looking like a kind of inverted "U" which is located in the abdominal cavity. The colon is distinguishable from the small intestine because it is much wider, with at regular intervals pockets (haustra) with hollows in between (plicae semilunares; embryo created by fold formations in the wall) to be seen and three longitudinal muscle bands. The colon runs from the appendix (caecum) in the lower right abdomen, first straight up (colon ascendant / ascending colon) to the liver. Then the colon turns sharply to the left (colon transversum / transverse colon). Under the spleen it bends back down (descendent colon / descending colon). Then the colon makes an s bend forward (colon sigmoideum or simply the sigmoid) and afterwards turns into the rectum (rectum).

In the appendix are 2 large mucosal folds (Bauhini doors), to prevent food flowing back to the small intestine. At the bottom of the appendix is a worm-shaped Appendix: the appendix vermiformis. When one speaks of "appendicitis", it is really this inflamed appendix. 

 Picture source: Uptodate.com

Just like the small intestine, the wall of the large intestine consists of 3 layers. From the outside inwards: the double layer of muscle, the layer of connective tissue and the layer of mucous membrane. The mucous membrane of the large intestine certainly has folds but there are no villi. The total surface area of the large intestine mucous membrane is much smaller (4m²) than that of the small intestine (150-200m²).

Picture source: Academie voor Mesologie

The most important function of the large intestine is to extract the fluid and salts from the excrement and
to reabsorb them into the body. As a result the excrement becomes thicker and is changed into a form that the body can eliminate easily. The large intestine produces mucous that acts as a lubricant. In the large intestine there are enormous numbers of bacteria, around 100 billion and more than 400 types. These are known as intestinal flora (see illustration here below). Intestinal flora consist of useful and harmful bacteria. In healthy flora these bacteria are in balance with one another. Intestinal flora are responsible for the fermentation
and breakdown of the content of the intestine, whereby matter is released which stimulates the movements of the large intestine. Healthy intestinal flora also protect us from infections. If the harmful bacteria gain the upper hand then we can fall ill.

Picture source: Quarks & Co

The large intestine (colon) is characterized by its haustra. These are folds of the intestine along the outside every few centimeters seems like cords. The haustrale contractions are slow movements and last 1 minute, and happen every 30 minutes. On the images below you can see how the stool (Latin: faeces) in the colon is formed. Mixing of the large-intestinal contents happens due to local contractions of the circular muscle layer (peristaltic). In the first picture you see segmentation movements. The stool is molded by these movements about every half hour (massive colon contractions caused by contraction of the longitudinal taeniae coli). This stool is not driven further into the anus. In the middle picture you see propelling contractions. Through waves of contractions and segmentation movements the stool is driven to the rectum (propulsion). In the last picture you see mass movements. They occur after a meal in the large intestine. Hereby a part of the relief is pushed to the lower part of the S-shaped colon and rectum. Haustral contractions ensure that the gut contents move back and forth, so that not only the outside of the relief is dehydrated, but also the inside.

Picture source: Reader's digest/ Goede spijsvertering

At the end of the large intestine there is a 12 cm long temporary storage place (rectum) where the excrement collects and is carried towards the anus. The 4 cm long anus is the closed end of the rectum. The skin round
the anus, just like the lips, is covered in sensory nerves. This opening is closed not by one but by two powerful muscles: the internal sphincter and the external sphincter. The internal sphincter is an involuntary muscle consisting of muscle cells which contract of their own accord. The outer sphincter is a voluntary muscle and thus consists of muscle cells which contract on command. Together they form a powerful door which can open and close the rectum.

Picture source: AAFP

As the rectum fills, so the excrement presses against the internal sphincter of the anus. As a result of this stimulation this sphincter begins to relax. At the same time the outer sphincter tightens. This tension and relaxation happens automatically and is known as the ‘involuntary reflex’ of the anus. This reflex gives rise to
an urge to go to the toilet: the body thinks it is time to get rid of its waste. The anus is still closed: the involuntary reflex does not cause the anus to open automatically, not even when the urge is strong. We can
relax the outer sphincter ourselves. So the reflex to empty the rectum is under our control.

Picture source: Medline plus

As a result of the digestive process the intestines produce gasses (flatus). Every day we produce about
a quarter of a litre of gas, 80% of which is breathed out via the blood and lungs. The rest leaves via the anus,
not necessarily making a sound when it does so.

 Picture source: Alles over je lichaam/Mieke de Haan

As you read our digestive system is a complex happening. It is no wonder that sometimes things go wrong. There are about 200 diseases which can actually happen, from harmless ailments such as a hemorrhoid, to more serious things like inflammation and cancer. 

Return to top

 

Urinary Tracts

As you have read above, the intestines take care of the digestion of food; the urinary tracts see to the transport of fluid. You take in fluid not only through drinking but also through eating. Some of the fluid remains
in the body while the remainder is used to remove waste materials and to regulate body temperature (perspiration). Sweat glands in the skin and urine production in the kidneys ensure the removal of waste products. We have two kidneys (see picture below), these kidneys filter 120ml of fluid every minute, and only
1 ml per minute of this becomes urine. This is the exact amount required to dissolve waste products and eliminate them from the body.

Picture source: UMC St Radbout

 

Urine is carried to the bladder by ureters. As you can see in the picture below, each kidney has its own ureter.

Picture source: Mayoclinic

 

The ureters are 25 to 35-cm-long ollow tubes which each end in the two top back corners of the bladder.
There are valves at the openings which open only one way to prevent the urine flowing back to the kidneys whenever the bladder is too full. The urine is collected in the bladder which is a sort of temporary storage
place. An empty bladder is about 7 cm long, and a full bladder can be 12 cm. The wall is made up of three
layers of smooth muscle fibres so the bladder can easily adapt itself to changes in content.

 

Picture source: Women's health matters

 

The sensitive spot in the bladder is the triangle between the urethra and the two ureters: the trigone. The urine trickles continuously into the bladder and at a given moment the bladder becomes so full that that this area is stretched and via the sensory nerves in this triangle you get the feeling that the bladder is filling up.

Between the urethra and the bladder there is a sphincter. When this relaxes the urine is released via the urethra. Women have a shorter urethra (about 4 cm) than men (about 20cm). The female urethra is almost straight and comes out at the front of the vagina. The male urethra is bent in two and comes out in the glans
of the penis which serves both to release urine and to transport semen.

Picture source: Strong Health